1. Field of the Invention
The present invention relates to a display device and a driver circuit of the display device, particularly to an active matrix display device having thin film transistors formed on an insulator and a driver circuit of the active matrix display device. More particularly, the present invention relates to an active matrix liquid crystal display device using a digital image signal as an image source and a driver circuit of the active matrix liquid crystal display device.
2. Description of the Related Art
In recent years, a display device having a semiconductor film formed on an insulator, particularly on a glass substrate, particularly an active matrix display device using thin film transistors (hereinafter referred to as TFTs) have been spreading. The active matrix display device using TFTs has several hundred thousands to several millions of TFTs arranged in matrix and performs an image display by controlling a charge of each pixel.
Further, as a recent technique, a technique relating to a polysilicon TFT for simultaneously forming a driver circuit in the peripheral portion of a pixel portion with a pixel TFT constituting a pixel is developing, which greatly contributes to miniaturization and lower power consumption of a device. Along with this, a liquid crystal display device has been becoming an essential device for a display portion of a mobile apparatus etc. in which the applied field has been remarkably expanding in recent years.
A schematic diagram of an active matrix liquid crystal display device of a normal digital system is shown in FIG. 14A. A pixel portion 1404 is arranged in the center of a substrate 1401. On the upper side of the pixel portion, a source signal line driver circuit 1402 for controlling source signal lines is arranged. On the right and left sides of the pixel portion, gate signal line driver circuits 1403 for controlling gate signal lines are arranged. Although symmetrically arranged on the right and left sides of the pixel portion in FIG. 14A, the gate signal line driver circuit 1403 may be arranged on one side. However, the arrangement on both sides is desirable from the viewpoint of driving efficiency and driving reliability of the liquid crystal display device. Input of signals to the respective driver circuits from the outside is conducted through flexible printed circuits (FPCs) 1405.
FIG. 14B is an enlarged view of a circuit diagram of a portion of 2×2 pixels surrounded by a dotted line frame 1406 in the pixel portion 1404 in FIG. 14A. One pixel has a source signal line 1451, a gate signal line 1452, a pixel TFT 1453, a liquid crystal 1454, and a storage capacitor 1455.
The source signal line driver circuit 1402 has the structure shown in FIG. 15, for example. The driver circuit shown as an example in FIG. 15 is a source signal line driver circuit corresponding to a 3-bit digital gradation display, which has a shift register circuit (SR) 1501, a first latch circuit (LAT1) 1502, a second latch circuit (LAT2) 1503, a D/A (digital/analog) converter (DAC) 1504 and the like. Note that although not shown in FIG. 15, a buffer circuit, a level shifter circuit and the like may be arranged if necessary.
The operation is simply described with reference to FIG. 15. First, the shift register circuit 1501 is input with clock signals (S-CLK, S-CLKb) and a start pulse (S-SP), and sampling pulses are sequentially output. Then, the sampling pulses are input to the first latch circuit 1502, and in accordance with this timing, digital image signals (digital data) also input to the first latch circuit 1502 are respectively held. Here, D2 is the most significant bit (MSB) and D0 is the least significant bit (LSB). In the first latch circuit 1502, after the completion of holding the digital image signals for one horizontal period, the digital image signals held in the first latch circuit 1502 are simultaneously transferred to the second latch circuit 1503 in accordance with the input of latch signals (latch pulses) in a return line period.
Thereafter, the shift register circuit 1501 is operated again, and holding of digital image signals for the next horizontal period is started. On the other hand, at the same time, the digital image signals held in the second latch circuit 1503 are converted into analog image signals in the D/A converter 1504. The digital image signal converted into an analog image signal is written into a pixel 1505 of one row in a state that the gate signal line is selected through the source signal line. This operation is repeated, and thus, the image display is conducted.
In a general active matrix liquid crystal display device, renewal of a screen display is conducted about sixty times per second in order to smoothly perform a display of moving images. That is, it is necessary that every time a digital image signal is supplied each one frame, write into a pixel is conducted. Even if the image is a static image, the same signal has to be continuously supplied every one frame. Thus, it is necessary that the driver circuit continuously and repeatedly performs the process of supplying the same digital image signal.
There is a method in which a digital image signal of a static image is once written into an external memory circuit, and then, the digital image signal is supplied to a liquid crystal display device from the external memory circuit every one frame. However, the external memory circuit and the driver circuit have to continuously operate in any case.
Particularly in mobile apparatuses, lower power consumption is greatly desired. Further, in the mobile apparatus, though it is mostly used in a static image mode, the external circuit, the driver circuit, and the like are continuously operated in a static image display as described above. Thus, this is an obstacle to the lower power consumption.